1. Field of the Invention
This invention relates to a semiconductor element provided with a junction made by laminating a second layer to a first thin layer. More particularly, to a semiconductor element having an electric junction the main part of which is formed from a polycrystalline silicon thin film semiconductor layer of high performance characteristics, reliability and stability.
2. Description of the Prior Art
Recently, the formation of a scanning circuit portion of an image reading device for use in image reading such as a one-dimensional photosensor made in a continuous length or a two-dimensional photosensor of an enlarged area, has been the subject of research. The formation of a driving circuit of an image display device utilizing liquid crystal (abbreviated as LC), electrochromic material (abbreviated as EC) or electroluminescence material (abbreviated as EL), and the formation of a light receiving element member of the photosensor and a switching circuit for the display device, has been investigated. It has been proposed to use a silicon thin film formed on a certain substrate as a fabrication material, corresponding in size to the increased area of such display portions.
Such a silicon thin film is preferably polycrystalline rather than amorphous for realization of a large scale image reading device or image display device improved speed and function. This is so despite the requirement that the effective carrier mobility (.mu.eff) of a silicon thin film, for example, a field effect transistor, as the base material for forming a light receiving portion and a scanning circuit portion of such a high speed, high function reading device or switching portion and driving circuit portion of image display device should be large. The .mu.eff of the amorphous silicon thin film obtained by an ordinary discharge decomposition method is at most 0.1 cm.sup.2 /V.sec. In addition, as DC voltage is applied to the gate, the drain current decreases and the threshold voltage of the transistor changes. Such changes after a period of time are signifcant and the stability of the device is poor.
In contrast, a polycrystalline silicon thin film has an effective carrier mobility, .mu.eff, far greater than an amorphous silicon thin film as is seen from the practical data as measured. Theoretically, it is quite probable that there can be prepared a polycrystalline silicon thin film having a further increased value of mobility .mu.eff beyond than that presently obtained.
However, at present the elements or devices produced by using polycrystalline silicon thin film as a base material according to various methods do not sufficiently exhibit the desired characteristics and reliability.